Field of Endeavor
The present application relates to polymer films and more particularly to free-standing ultra-thin polymer films.
State of Technology
This section provides background information related to the present disclosure which is not necessarily prior art.
The Prior Art method of preparing ultra-thin polymer films is illustrated in FIGS. 1A, 1B, 1C, and 1D of the drawings, The Prior Art method is designated generally by the reference numeral 100. The Prior Art method 100 includes the addition of a release layer. The release layer is a sacrificial layer that is dissolved when the film is removed from the substrate. The films are removed from the substrate by immersing the substrate into water, which separates the film from the substrate, and the film will float on the water surface. FIG. 1A shows a silicon wafer 102. The silicon wafer 102 is the starting point in the Prior Art method 100 of preparing ultra-thin polymer films. Referring now to FIG. 1B a sacrificial layer 104 is shown on top of the silicon wafer 102. The sacrificial layer 104 can be made of sputtered salt, soap or other release agents, such as Victawet. Referring now to FIG. 1C a formar film 106 is show on top of the sacrificial layer 104 the silicon wafer 102. Referring now to FIG. 1D the step of removing the formar film 106 from the substrate 102 is illustrated. This is accomplished by immersing the substrate 102 into water 110 in a water bath 108. This dissolves the water soluble sacrificial layer 104 and separates the film 106 from the substrate 102.
The Prior Art method of preparing ultra-thin polymer films has several disadvantages. The substrate preparation can introduce roughness, especially in the case of sputtered liftoff layers, which can be on the order of several nanometers, which leads to film non-uniformity and becomes more severe as the film thickness is reduced below 20 nm. Sacrificial liftoff layers can contaminate the film and decrease the strength of the film. At thicknesses lower than 30 nm, the liftoff of the film from the substrate becomes impossible for some preparations. Another drawback is the inability to produce large free-standing films. This inability is related to the shape of the holder, the liftoff technique itself, and the properties of the polymer that is used to produce the thin film. Large films will often tear when lifted out of the water, and in some cases they tear while drying.